Examine source code of Firefox Stereogram

{
  "theme": {
    "images": {
      "theme_frame": "215.jpg"
    },
    "colors": {
      "frame": "#000000",
      "tab_background_text": "#000000"
    }
  },
  "version": "2.0",
  "name": "Firefox Stereogram",
  "manifest_version": 2,
  "description": "This header is a stereogram (one of those 3D puzzles you have to focus your eyes to see) I tested it on my screen which is 1366x768 and while my toolbar buttons made it a little more challenging, it actually works. Its a text image. Focus on the right of the header."
}